Many oil refineries recover valuable products from the heavy residual hydrocarbons (commonly referred to as resid or residuum), which remain following initial refining, by a thermal cracking process known as delayed coking. Generally, the delayed coking process involves heating the heavy hydrocarbon feed from a fractionation unit, then pumping the heated heavy feed into a large steel vessel commonly known as a coke drum. The unvaporized portion of the heated heavy feed settles out in the coke vessel where the combined effect of retention time and temperature causes the formation of coke. Vapors from the top of the coke vessel, which typically consist of steam, gas, naphtha and gas oils, are returned to the base of the fractionation unit for further processing into desired light hydrocarbon products. The operating conditions of delayed coking can be quite severe. Normal operating pressures in coke vessels typically range from 25 to about 50 pounds per square inch and the heavy feed input temperature may vary between 800° F. and 1000° F.
Coke vessels are typically large, cylindrical vessels commonly 19 to 30 feet in diameter and two to three times as tall having a top head and a funnel shaped bottom portion fitted with a bottom head and are usually present in pairs so that they can be operated alternately. Coke settles out and accumulates in the vessel until it is filled to a safe margin, at which time the heated feed is switched to the empty “sister” coke vessel. Thus, while one coke vessel is being filled with heated residual oil, the other vessel is being cooled and purged of coke (between 500 and 1200 tons) formed in the vessel during the previous recovery cycle. The full vessel is isolated, steamed to remove hydrocarbon vapors, cooled by filling with water, drained, opened, and the coke is removed. The drums typically operate on a cycle, switching every 12–30 hours.
Coke removal, also known as decoking, begins with a quench step in which steam and then water are introduced into the coke filled vessel to complete the recovery of volatile, light hydrocarbons and to cool the mass of coke. The vessel is then vented to atmospheric pressure. Decoking is accomplished at most plants using a hydraulic system consisting of a drill stem and drill bit that direct high pressure water jets into the coke bed. To cut coke in this manner in conventional systems the top and bottom heads of the vessel must be removed. A rotating combination drill bit, referred to as the cutting tool, is about 18 inches in diameter with several nozzles and is mounted on the lower end of a long hollow drill stem about 6 inches in diameter. The drill bit is lowered into the vessel, on the drill stem, through a flanged opening at the top of the vessel. A “bore hole” is drilled through the coke using the nozzles which are angled approximately 60 degrees down from horizontal and ejects water at pressures in the range of 2600 to 3600 psig. This creates a pilot bore hole from about 3 to 6 feet in diameter for the coke to fall through.
When the initial bore hole is complete, the drill bit is then mechanically switched to at least two (2) horizontal nozzles in preparation for cutting the “cut” hole, which extends to the full drum diameter. The nozzles shoot jets of water horizontally outwards, rotating slowly with the drill rod, and those jets cut the coke into pieces, which fall out the open bottom of the vessel, into a chute that directs the coke to a receiving area. At some plants the hydraulic drill is raised slowly up from the bottom the entire vertical height of the coke mass, at others the drill is lowered from the top through the mass and at still other plants the coke mass is first cut from the bottom cone of the vessel and the remainder is cut from the top of the vessel. In any case, the cut coke falls out the opening at the bottom of the vessel into a coke chute system. The drill rod is then withdrawn out the flanged opening at the top of the vessel. Finally, the top and bottom of the vessel are closed by replacing the head units, flanges or other closure devices employed on the vessel unit. The vessel is then clean and ready for the next filling cycle with the heavy hydrocarbon feed.
The process of removing and replacing the removable top head and bottom units of the vessel cover is called heading and unheading or deheading. It is very dangerous work, with several safety hazards associated with the procedures. There have been fatalities and many serious injuries. There are significant safety hazards from possible exposure to high pressure water jets, steam, hot water, fires and repetitive stress associated with the manual unbolting work necessary with conventional systems. Accordingly, the industry has concentrated most of their technological improvements in the field of coking to minimize the safety hazards associated with unheading procedures. The once manual deheading procedure has evolved into semi-automatic to fully automatic unheading systems to improve the efficiency of the coking process and alleviate the safety concerns with heading and deheading of coker drums.
The deheading systems have primarily focused on the bottom of the coker unit for safety and economical reasons. There are two typical and commonly used methods to move the bottom head out of the way of the falling coke. The first is to completely remove the head from the vessel, perhaps carrying it away from the vessel on a cart. The other way of “removing” the bottom head is to swing it out of the way, as on a hinge or pivot, while the head is still coupled to the vessel as in U.S. Pat. No. 6,264,829. These systems all use a manual or semi-automatic bolting system that must be uncoupled with every decoking cycle.
Coker vessel top deheading systems are similar to the bottom deheading systems for the coker except they are generally smaller in size. Several different types of mechanical top head systems are used. For example, one type of mechanical top unheading system in common use is referred to as a “plate blind” which serves to open and close the coker drum top. It functions to maintain a positive isolation of the drum's interior contents from exposure to the outside atmosphere in the closed position during the coking or feeding part of the coking cycle. In preparation for decoking, the “plate blind” is removed, exposing the drum contents to the atmosphere. To remove coke a second mechanical top head is used to setup the drill assembly and contain the coke particulant, steam, and hot water during drilling and decoking of the coke vessel. After this device is attached, the drum contents are no longer directly exposed to the atmosphere.
Several additional U.S. patents disclose similar systems. U.S. Pat. No. 5,022,799 describes a coker deheading apparatus and methods for alignment and mounting of the drill stem without direct operator contact. The drill stem is provided in the vicinity of the cutting tool, with a drill stem guide carried on a slidably mounted plate. U.S. Pat. No. 5,092,963 describes an automated top head and drill stem guide assembly which is adapted for remotely operated pivotal removal and replacement of a cover unit from the top of a vertically oriented vessel such as a coking drum. The invention includes a flanged connector unit, which is attachable to the flanged vessel top opening, a top head cover device, and an automated drill stem guide device. These elements are each pivotally attachable to the flanged connector unit, which flanged connector unit is attached pressure-tightly to a top flange of the vessel such as with a coking drum. U.S. Pat. No. 5,259,930 discloses an automated top head cover and stem guide assembly adapted for covering a top opening in vertical vessels such as coking drums, and a method for remotely operating the assembly. The top head assembly includes a flanged connector unit attached pressure-tightly to a top flange opening of a coking drum and a top cover device including a cover unit pivotally attached to the flanged connector unit, so that the cover unit can be pivotally lifted and moved aside. A drill stem guide device is also pivotally attached to the top flange unit, so that a drill stem guide unit can be pivotally moved downwardly into place over the top flange unit opening after a drill stem member is inserted into the vessel. U.S. Pat. No. 5,417,811 discloses a closure device adapted for attachment onto the upper head of a coking drum to prevent hot vapors escaping during drum decoking operations. The closure device comprises a cylindrical shaped housing having a rotatable ball valve and horizontal elongated stem provided in its lower portion, and a cover unit including a packing gland provided at the housing upper end. A rotatable cutting tool provided within the housing above the ball valve has a drive rod extending upwardly through the cover unit packing gland. During operations, the ball valve is opened and the drive rod and its attached cutting tool can be extended downwardly through the ball valve cylindrical-shaped opening into the coking drum and rotated, so as to cut and dislodge coke deposited in the drum.
U.S. Pat. No. 6,228,225 discloses a semi automatic coke vessel deheading device. This invention replaces the blind flange or the top of the coker drum with a clamshell assembly. The deheader device generally comprises a frame, a rotatable bridge, a lift assembly, a rotational assembly, and a plurality of actuators for opening each assembly. The deheader device lifts and rotates the blind flange away from the coker drum in order that the clam shell assembly may be attached to the coke drum prior to the coke cutting process. The blind flange and clamshell assembly are attached on opposite ends of the rotating bridge which can be rotated about a vertical axis of the deheader device through and angle up to 180 degrees.